1. Field of the Invention
The present invention relates to lithographic apparatus and to a device manufacturing method.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
As lithographic apparatus are used to manufacture devices with ever smaller critical dimension (CD), i.e. the dimension of a feature or features in which variations will cause undesirable variation in physical properties of the feature, in lithography shrinks, such as the gate width of a transistor, it is often necessary to use a high numeric aperture (NA), e.g. of greater than 1, to achieve the required high resolution. However, the higher the NA the lower the depth of focus so that it is desirable to be able to adjust the NA setting of an apparatus to enable printing of layers where depth of focus is more important than resolution or line width.
The NA of a projection system can be adjusted by providing a carousel arrangement with a plurality of aperture plates or an adjustable iris diaphragm in the pupil plane of the projection system. However, carousel arrangements are bulky and provide only a limited number of NA settings so that irises are more commonly used. These include an arrangement of leaves, e.g. 4 or 6 or more, arranged around a central aperture and pivotable to vary the diameter of that aperture. As a finite number of leaves is used, the aperture is inevitably polygonal (with curved sides) rather than perfectly circular except at one NA setting so that at other NA settings the NA of the projection system will not be uniform in all directions. The NA uniformity is therefore only acceptable for a relatively small range of aperture diameters and hence NA settings. Increasing the number of plates increases the range over which acceptable NA uniformity can be obtained but increases the bulk and complexity of the iris. An example of an iris diaphragm is disclosed in U.S. Pat. No. 6,445,510.